A refractive index distribution type optical element of which a refractive index varies corresponding to a parabolic distribution in a radial direction, for example a light focusing lens, even when the lens has a flat face, is provided with the same imaging function as a spherical lens. The optical device has an advantage that it is easy to produce a lens having an extremely small diameter and a single focus. The refractive index distribution type optical element is widely applied for an optical head of an optical device including an optical printer, a facsimile, a laser printer, and the like. Such refractive index distribution type optical elements include a refractive index distribution type rod lens, a refractive index distribution type fiber and the like.
To produce the refractive index distribution type optical element, an ion exchange method of soaking a glass rod into a molten salt to distribute the refractive index is widely applied. For example, a rod lens having distribution of the refractive index can be produced by soaking a glass rod containing cations including Li.sup.+, Tl.sup.+, Cs.sup.+, Ag.sup.+ and the like into the molten salt including sodium nitrate and potassium nitrate and the like to exchange the cations contained in the glass for those in the molten salt.
In stead of using the molten salt, a rod lens containing the core/cladding structure having distribution of the refractive index can be produced by heating it to exchange the cations between the rod glass and the layer of the cladding glass while or after forming the core/cladding rod lens which is formed due to covering the aforementioned glass rod with a layer of the cladding glass containing sodium ion or potassium ion. The core/cladding rod lens can be produced with the double crucible method, pipe rod method and the like.
When the rod lens is produced with the ion exchange method, since the ion exchange process progresses principally due to diffusing phenomenon, the refractive index often departs from the parabolic distribution (quadric distribution) mainly around the side face thereof FIG. 1 shows the sectional view of the core/cladding rod lens and the distribution of the refractive index thereof. According to FIG. 1, the lens has a circular cross section and the lens contains the core in the center portion thereof and the coaxial cladding surrounding the core. A curve of distribution of the refractive index is shown below the sectional view of the lens. The abscissa r expresses the radial distance from the center of the core and the ordinate n expresses the refractive index. The glass composition of the core and the cladding before the ion exchange treatment is determined so that the refractive index continuously varies at the boundary between the core and the cladding after ion exchange treatment especially when the thickness of the layer of the cladding is made relatively larger in comparison with the diameter of the core.
When the data including the image is transmitted, beams passing through portions having the refractive index departing from the regular parabolic distribution in the vicinity of the side face of the core cause optical distortion and aberration to deteriorate the optical properties of the refractive index distribution type rod lens. And the light entering the refractive index distribution type rod lens through parts adjacent to the side face thereof (generally called as the flare light) also deteriorate the optical properties of the refractive index distribution type rod lens.
In order to prevent deterioration of the optical properties of the lens and entrance of the flare light, the Japanese Patent Publication S63-301901A makes disclosure of a method of preventing the entrance of the flare light in that a light absorbent layer of the glass including colorants consisting of metal ion including Mn, Cr, Co, Ni, Fe, Cu, Ag, Ti, Pb, Ru, Cd, V, Mo and the like to prevent the entrance of the flare light is formed in the cladding while producing the refractive index distribution type optical element by soaking the core/cladding glass rod respectively containing the cation of Li.sup.+ and the like into the molten salt, for example the molten salt comprising the sodium nitrate, for the determined periods to exchange the lithium ion contained in the core/cladding glass rod for the sodium ion existing in the molten salt. The colorant used in an example of the invention of the Japanese Patent Publication S63-301901A which includes MnO, CoO or a combination of CoO and MnO is available for the optical device employing the monochromatic light for the illuminant, while it is insufficiently provided with the resolution when used for the optical device employing the white light for the illuminant.
In the Japanese Patent Publication H10-139472A proposes a refractive index distribution type optical element having a cladding glass layer consisting of a light absorbent glass including CoO, MnO and Cr.sub.2 O.sub.3 as the colorant, e.g. 1.5 wt. % CoO, 1.0 wt. % MnO and 0.4 wt. % Cr.sub.2 O.sub.3, so as to prevent the entrance of the flare light and to improve the external diameter precision. The colorant easily causes devitrification to the core and the cladding glass, and it has limited applications for the glass composition of the core and the cladding glass.